A number of methods have been developed to determine the state of charge of batteries or individual battery cells. One method commonly used in connection with flooded or excess electrolyte batteries is the measurement of the specific gravity of the electrolyte. Such a measurement is accomplished by the use of a hydrometer which may be situated inside the battery itself. A manual hydrometer can also be used to measure the specific gravity of the electrolyte, and thus determine the state of charge. Use of a manual hydrometer, however, normally requires that the battery be disconnected and that a significant quantity of electrolyte be removed through a battery fill cap or other opening. This may result in electrolyte spillage and time-consuming clean-up operations. Because a hydrometer requires free electrolyte, a hydrometer cannot be used in starved electrolyte batteries, which contain no free electrolyte, or in sealed, maintenance-free, lead-acid batteries.
Various electronic devices are also frequently used to measure the state of charge of battery cells. Although such devices are generally reliable, they are often quite expensive. Electronic state of charge indicators generally measure variables such as the open-circuit voltage of the battery, or the voltage under load. These measurements are then compared with a reference scale to determine the state of charge of the battery. The operation of these electronic devices may require the use of an external power source, or the battery cell itself may supply the power to monitor the state of charge. This latter arrangement, however, contributes to cell discharge.
The state of charge of a battery may also be determined in relation to the differential swelling of dissimilar plastic materials placed within the battery. Such materials swell due to the vapor pressure of the air in the headspace above the electrolyte, causing a dimensional difference between the dissimilar materials. This differential can be measured and used to determine the state of charge of the battery. Indicating devices made of dissimilar materials, however, are generally sensitive to temperature and movement, and are therefore often unreliable as a result of hysteresis.
Other devices used to indicate the state of charge of a battery include various types of color-coded display systems. For example, a cell may contain an indicating device made of litmus paper which changes color in response to a change in the pH of the free electrolyte, thereby indicating a change in the state of charge. Color-coded charge indication systems are typically designed and located within the battery so that the litmus paper is in direct contact with the electrolyte. Such indicating devices are generally located adjacent to or in the walls of the battery container, and often include a window so that color changes in the litmus paper are visible and may be compared with a reference scale.